Atmospheric corrosion of outdoor bronze: mechanism of decay and conservation strategies

Application of protective coatings is the most widely used conservation treatment for outdoor bronzes. Innovative, eco-friendly and non-hazardous coatings are currently needed to enlarge the batch of efficient advanced formulations for conservation of outdoor bronze monuments. To fulfil this need, the M-ERA.NET European research project B-IMPACT (Bronze-IMproved non-hazardous PAtina CoaTings) aimed at assessing the protectiveness of innovative organic coatings for historical and modern bronze monuments. In this PhD thesis, elaborated in the framework of B-IMPACT project, the assessment of novel, eco-friendly and non-hazardous protective coatings was carried out. They were compared to Incralac®, one of the most used protective coating in the conservation practice. A multi-analytical approach was adopted considering electrochemical evaluation, artificial ageing and metal release, as well as the morphological and structural evolution of the coated surfaces induced by the simulated outdoor exposure. The best performance as a non-toxic alternative to the conventional Incralac® treatment was exhibited by a fluoroacrylate blended with methacryloxy-propyl-trimethoxy-silane (FA-MS) applied on patinated Cu-Sn-Zn-Pb bronze and a 3-mercapto-propyl-trimethoxysilane (PropS-SH) coating applied on Cu-Si-Mn bronze, black patinated by “liver of sulphur” procedure (based on the use of K2S aqueous solution). Finally, this PhD thesis also presents a X-ray Photoelectron Spectroscopy (XPS) and Electron Microscopy (FEG-SEM+FIB) characterization of a thin PropS-SH film on casting bronze. In particular, PropS-SH was firstly studied on pure Cu and Sn and then on bronzes with increasing alloy additions. In the case of coated quinary bronze, the free surface of samples was also analysed by High Resolution Photoelectron Spectroscopy using Synchrotron Radiation (HR-SRPES) at ANTARES (Synchrotron SOLEIL), which offers a higher energy and lateral resolution than conventional instruments. By compiling complementary spectroscopic and imaging information, a deeper insight into the interactions between the protective coating and the bronze substrate was achieved.